Applicant claims the right of priority under 35 U.S.C. xc2xa7 119 based on patent application No. MI97A 000764, filed Apr. 3, 1997, in Italy.
The present invention relates to an extrusion method for manufacturing tread bands for vehicle tires, as well as an extrusion apparatus for carrying said method into effect, said apparatus being of the type comprising: at least one primary extrusion head for feeding at least one primary elastomer material; at least one auxiliary extrusion head for feeding at least one auxiliary elastomer material; a preforming device having at least one primary preforming channel connected to the primary extrusion head; an extrusion die operatively disposed downstream of the preforming device and having an extrusion outlet located at the end of the primary preforming channel and defining an extrusion path therewith.
It is known that in the field of tire manufacture for vehicle wheels, the tread band is obtained by extrusion of a workpiece in the form of a continuous strip which is subsequently cut into lengths of predetermined measure, each of which is adapted to be associated with the other tire components during an assembling step provided in the production process: during this step all workpieces forming the tire are fit together at room temperature, said pieces, being cold too, since they come from a storage magazine.
In more detail, the stretch forming the tread band is applied crownwise either to the carcass, usually already toroidally shaped, directly on the carcass ply or plies, or to the belt structure previously wrapped over said carcass, in such a manner that the opposite side edges of the tread band join the end flaps of the tire sidewalls. Depending on planning choices, these end flaps are either applied to the carcass plies so as to be enclosed between the latter and the opposite side edges of the tread band, or directly applied in superposed relationship to the opposite side edges of the tread band itself.
Usually, the type of blend used in making the tread band depending on the required performance (wear resistance and grip on a dry and wet road surface, for example) has such chemical features that it appears difficult to obtain an efficient cold adhesion to the elastomers employed in making the carcass and belt structures and the tire sidewalls. Under this situation, in order to eliminate the necessity to submit the tread band to processes involving application of a petrol solution in order to restore the necessary stickiness during the tire assembling step, in many cases use is made of a so-called xe2x80x9cskim coatsxe2x80x9d to be applied to the lower surface of the tread band and intended to get into contact with the carcass ply or belt structure. This skim coat is essentially made up of a thin layer of natural rubber-based blend that, being applied to the tread band when it is still hot immediately after its emerging from the extrusion head, can be efficiently anchored to the tread band blend and supplies an appropriate substrate for cold adhesion to the carcass without resorting to surface processes involving application of a petrol solution.
Likewise, in the case of overlapping of the sidewalls on the opposite side edges of the tread band, the side edges of the tread band as well are intended to be provided with two border inserts, the so-called xe2x80x9cmini-sidewallsxe2x80x9d also made of an elastomer material having such features that an efficient adhesion between tread band and sidewalls is ensured during the assembling step of the tire, usually by adopting the same blend for the sidewalls and mini-sidewalls in order to facilitate cold adhesion.
Application of mini-sidewalls to the tread band takes place by a co-extrusion process, a hot process therefore, which consists in employing on the same extrusion unit intended for the tread band formation, a second extrusion head supplying the blend for the mini-sidewalls along two auxiliary feeding channels converging at the opposite side portions of an extrusion outlet, in a direction substantially parallel to the course of the primary feeding channel through which the elastomer material intended for forming the true tread band is fed.
In the present state of the art, unlike the mini-sidewalls, the skim coat is produced in a separated unit, generally a calender, and rolled onto bobbins; each bobbin is then mounted on an appropriate unwinding device from which the skim coat is taken out for application to the tread band already provided with the optional mini-sidewalls, while said tread band is moving away from the corresponding extrusion unit.
Usually, since the skim coat is applied to the lower surface of the tread band, the unit carrying out unwinding of the skim coat is disposed under the whole tread band extrusion plant that consequently must be placed at a raised position, which will bring about complications in the plant structure and difficulties in acceding to all parts thereof.
In order to eliminate the requirement for use of a separated unit for skim coat production and a bobbin unwinding unit to deliver the skim coat to the tread band extrusion unit, attempts have been made to extrude the skim coat together with the tread band, in the same manner as pointed out with reference to mini-sidewalls, by adopting a co-extrusion method already known and successfully used for producing tread bands made up of two blends, where thickness of the lower blend layer is about half the thickness of the whole tread band.
The Applicant has however found that this co-extrusion method gives poor results when, in an attempt to make the skim coat, thickness of the underlying blend layer is reduced to values as low as two millimeters or less. Actually, in order to obtain very reduced thicknesses by extrusion, of 2 mm for example, due to swelling of the blend on coming out of the extrusion device, dies having apertures of about 1 mm are to be adopted and consequently possible blend impurities or dust may be entrapped into the die, thereby creating longitudinal scratches and therefore size irregularities in the extruded product or extrusion.
In addition, under such operating conditions checking of the blend flow rates at the different points of the extrusion section is very difficult and, therefore, sufficient assurances in terms of even and precise thickness of the skim coat and repeatability of results can be hardly given.
It is known that these difficulties are not present in calenders.
In accordance with the present invention it has been found that making the tread band provided with a skim coat in a co-extrusion process in which the skim coat itself has excellent qualities in terms of thickness evenness and thickness control is made possible if the blend intended for forming the skim coat is injected into the extrusion path of travel of the tread band in a direction perpendicular to, or in any case having a high angle of incidence with respect to the flow of the blend forming the tread band itself.
In a first aspect, the present invention relates to an extrusion method to make tread bands for vehicle tires comprising the steps of: conducting at least one primary elastomer material along an extrusion path comprising a primary preforming channel connected to an extrusion outlet the shape of which substantially corresponds to the required transverse section of the extruded product, conveying at least one auxiliary elastomer material to the extrusion path longitudinally passed through the primary elastomer, said conveying being carried out in a direction transverse to the movement direction of the primary elastomer longitudinally crossing the extrusion path, in such a manner that the auxiliary elastomer is distributed in the form of a thin layer over at least one surface of the extruded product being formed along the extrusion path.
In more detail the auxiliary elastomer material is conveyed to the extrusion path at a point immediately upstream of, or substantially close to the extrusion outlet.
Preferably, the conveying transverse direction of the auxiliary elastomer is substantially perpendicular to, or in any case forms an angle included between 30xc2x0 and 160xc2x0 relative to the movement direction of the primary elastomer along the extrusion path.
In a possible embodiment, the method in question further comprises the step of conveying at least one secondary elastomer to a pair of secondary preforming channels converging at two opposite side portions respectively of the extrusion outlet, in such a manner that two border inserts are formed on the tread band and extend along the opposite edges of the tread band itself.
Advantageously, the thin layer is applied to the lower surface of the tread band which is such arranged as to come into contact with a tire carcass on which said tread band will be employed.
Still in accordance with the present invention, the above method is carried into effect by an extrusion apparatus to make tread bands for vehicle tires, characterized in that it further comprises at least one auxiliary feeding channel connected to the auxiliary extrusion head and converging on the extrusion path in a direction transverse to the longitudinal extension of the latter.
In more detail, the auxiliary feeding channel converges on the extrusion path substantially at said extrusion outlet, preferably in a direction forming an angle included between 30xc2x0 and 160xc2x0, and more preferably of 90xc2x0 relative to the longitudinal extension of the extrusion path.
In a preferential solution of the invention, the auxiliary feeding channel has a connection length formed in the preforming device which terminates at said auxiliary extrusion head, as well as an end length formed between the preforming device and the extrusion die. Said end length preferably is defined by a surface hollow formed in the extrusion die.
Advantageously, said preforming device comprises a preformer-holding drawer and a preformer operatively housed in said drawer, said primary preforming channel being formed between said preformer and preformer-holding drawer, and said auxiliary feeding channel being at least partly formed in the drawer.
In accordance with a second aspect of the invention, which is valid in itself also independently of the presence or not of the auxiliary feeding channel, the preformer is comprised of two end portions disposed in mirror image relationship and at least one central insert removably engaged between said two end portions, said central insert being interchangeable so as to adapt the width of the primary preforming channel to the width of the extrusion outlet arranged in said die.
Centering means comprising, for example, a keyway in the central insert of the preformner and a centering key in the preformer drawer, enable the preformer to be fixed at a centered position relative to the drawer.
In a possible embodiment, the apparatus in question further comprises at least one secondary extrusion head to feed at least one secondary elastomer material, said preforming device having a pair of secondary preforming channels connected to said third extrusion head and converging at opposite side portions of said extrusion outlet respectively, so as to form two border inserts extending along the opposite edges of the tread band thus made.
In a third aspect, the present invention relates to an extrusion method to make tread bands with an elastomer insert, comprising the steps of:
conducting a primary elastomer flow along an extrusion path comprising a primary preforming channel connected to an extrusion outlet of a shape substantially corresponding to the required transverse section of the extruded product;
opening an empty region, adapted to be filled with another material, into the primary elastomer flow,
conveying at least one auxiliary elastomer material to the extrusion path longitudinally passed through by the primary elastomer, said conveying being carried out in a direction transverse to the movement direction of the primary elastomer longitudinally passing through the extrusion path, in such a manner that the auxiliary elastomer is homogeneously distributed in the empty region opened in the primary elastomer flow.
In accordance with another aspect, the present invention relates to an extrusion apparatus to manufacture tread bands for vehicle tires comprising an elastomer insert comprising:
at least one primary extrusion head to feed at least one primary elastomer material,
at least one auxiliary extrusion head to feed at least one auxiliary elastomer material,
a preforming device having at least one primary preforming channel connected to the primary extrusion head,
an extrusion die operatively disposed downstream of the preforming device and having an extrusion outlet disposed at the end of the primary preforming channel and defining an extrusion path therewith, characterized in that it further comprises:
at least one auxiliary feeding channel connected to the auxiliary extrusion head and converging on the extrusion path in a direction transverse to the longitudinal extension of the latter, and
means for opening an empty region adapted to be filled with another material, in the primary elastomer flow, and
means for enabling the auxiliary elastomer material coming from the auxiliary feeding channel to flow beyond the extrusion outlet and occupy said empty region.